Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view of a display screen or portion thereof showing a first embodiment of the claimed design; and,

FIG. 2 is a front view of a display screen or portion thereof showing a second embodiment of the claimed design.

The outermost broken-line rectangle depicts the perimeter of a portion of a display screen and forms no part of the claimed design. The remaining broken lines depict portions of a graphical user interface and form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, substantially as shown and described. 